Generally, two-wire loop process IO transmitters powered from the two-wire loop are used, to communicate between a process control and input and/or output (IO) field-devices, such as actuators and sensors, via the two-wire loop of a two-wire process control loop for controlling and/or monitoring IO points of the process.
In this regard the IO transmitters are adapted to operate on a two-wire loop being of a communication type in accordance with Foundation Fieldbus or with Profibus-PA but is not limited thereto.
Based thereon, the two-wire loops are commonly used for connecting a number of IO transmitters for facilitating the control and monitoring of certain IO points of an industrial process by IO field devices connected to the transmitters provided with respective IO interface ports. Thereby, the transmitters receive their power from the two-wire process control loop and being adapted to communicate over the two-wire process control loop with a central controller, such as a host. Thus, the two-wire loop is designed such that a transmitter receives its power from the two-wire loop as well as communicates on the two wire loop, wherein the two-wire loop is designed such that communication may occur without disrupting the provision of power to all transmitters attached to the loop. For facilitating the controlling and monitoring of IO points of a such process the transmitters are usually placed near the real industrial process and provide access to a plurality of process variables associated with IO field devices connected to the IO interface ports of the transmitters by transmitting digitized data over the two-wire loop to the central controller typically located at a greater distance away from the process, for example in a control room, than the transmitters, as it is schematically depicted in the attached FIG. 5.
FIG. 5 shows three transmitters all of which are connected to a common two-wire loop of a central controller located far away in a control room. Each of the transmitters includes an IO field device interface port to interface with a certain IO field device and to operate and/or transmit electrical signals relating to physical process conditions at the respective IO point that are to be controlled and monitored. In this regard, an IO field device can be an actor or sensor, such as a limit switch, a valve controller, a heater, a motor, a level indicator and an other IO field device. Hence, the nature of the data operated and/or transmitted by such a transmitter are for example temperature data, level data and flow data received from an IO field device being for example an input sensor field device to which the respective interface port is connected, and also can be of discrete inputs and outputs such as for example “stop data” and “go data” related for example to an IO field device being an output actor field device, for example motors. According to the state of the art, each of the three transmitters has its own power circuit integrated to receive power via the two-wire loop.
The more the central controller is located away from the real industrial process, there is the need to build up long cable runs in two-wire loop installations. Due to cable resistance however and based on the current draw and operating voltage of each of the transmitters necessary to control and monitor the IO points, the maximal length the cable runs are limited.
In addition, a further limitation in two-wire loop systems is given by the maximal number of IO transmitters that can be added to a single two-wire loop. Usually, a central controller or host and the process system controlled thereby have a practical limit of the number of IO transmitters that can be connected therewith. Such a practical limit is typically about sixteen connectable transmitters. As a consequence, this constraint also is limiting the number of IO field or process points that a single two-wire loop can capture, i.e. control and monitor, by IO field devices connected with the IO transmitters. Again as a result, several manufacturers offer IO transmitters with multiple IO interface ports multiplexed for data transmission over common channels by a single IO transmitter. A such IO transmitter is schematically shown for example in the attached FIG. 6. FIG. 6 is depicting a single IO transmitter connected to a two-wire loop of a two-wire process loop for receiving power from and for communicating with the central controller and having a multiplexer for multiplexing eight IO interface ports on a single channel as well as a single or common power circuit for wholly powering all of the eight IO interface ports with power received from the two-wire loop and a communication circuit for performing the entire communication. Even the incorporation of more than one multiplexer into a single IO transmitter having a single or common power circuit for wholly powering the entire transmitter is possible. Such a single IO transmitter having a single or common power circuit for wholly powering the entire transmitter described for example in U.S. Pat. No. 6,574,515.
EP 2053 697 and US 2009/0104814 describe common backplane type connection systems.
Depending on the adaptation of a respective IO transmitter to operate by means of a certain protocol, such as based on a communication protocol type according to Foundation Fieldbus or according to Profibus-PA but even on an other communication protocol type, usually the respective protocol used requires that a transmitter maintain a constant current draw from the two-wire loop. Thus, as existing two-wire loop-powered IO transmitters for interfacing with IO field devices contain a predefined number of IO ports, a fixed power consumption from the two-wire loop is needed. This however also limits the maximal length of cable runs.
Additional limitations are unnecessarily imposed on the user by existing IO transmitters often resulting in inefficient system designs.
For example, in case an IO transmitter is constructed as being a two-wire loop powered valves coupler and may have four IO interface ports providing the capability to interface with four valves, then a such valve coupler according to the state of the art draws the power from the two-wire loop necessary to interface with all four valves. Many industrial process applications however, often have more or less than four valves. As a first consequence, in case of only three valves have to be interfaced, the valve coupler nevertheless will draw the current necessary to interface with four valves regardless of whether or not a forth valve has to be interfaced by the IO transmitter. Thus in this case the described IO transmitter is inefficient for use.
In a case of an industrial process application however, according to which five valves for example have to be interfaced, a second transmitter of the afore-described type is needed in order to connect all five valves to the two-wire loop. In addition to the inefficiency of now powering eight valve interface ports instead of only five as needed, these two IO transmitters have to be commissioned on the central controller for operating the valves, thus increasing the number of IO transmitters connected to the controller. Because of the practical limit impost by the controller relating to the maximal number of IO transmitters that can be added to the two-wire loop, the IO transmitter described, i.e. having four IO valve interface ports, is not ideal matched even in such a case.
As a consequence, in today's large process installations, inefficiencies and challenges can result in unnecessary complexity and costs.